The present invention relates to an electromagnetic bearing for magnetically supporting a to-be-supported body in suspension, and more specifically to an electromagnetic bearing improved in its capability of adjusting the position of the to-be-supported body and reduced in the loss of magnetic energy.
Electromagnetic bearings are conventionally known as bearings for supporting an object such as a rotating body, in which the object is floated by magnetic force so that it has absolutely no contact with a stationary part. Because there is no contact, these magnetic bearings have various advantages: no mechanical abrasion, high durability, no mechanical noise, high-speed rotation, etc.
One such magnetic bearing is described in, for example, Japanese Patent Disclosure No. 19844/83. In this magnetic bearing, as shown in FIG. 1, a cylindrical rotating body or supported body 12 is disposed in a case 10. A cylindrical member 14 of the supported body 12 is formed of a nonmagnetic material. A pair of ring-shaped members 16 is attached individually to the upper and lower portions of the inner peripheral surface of the cylindrical member 14. The ring-shaped members 16 are formed of a highly permeable material. Inside the supported body 12, a magnetic supporting means 20 is set on a base 22 at the bottom of the case 10. Groups of magnetic poles 26 and 30 are formed on the upper and lower end portions, respectively, of a yoke 24 of the magnetic supporting means 20. Each magnetic pole group 26 or 30 includes four magnetic poles 26a, 26b, 26c and 26d (shown in FIG. 2) or 30a, 30b, 30c and 30d (only 30a and 30c are shown in FIG. 1) which project from the center of the yoke 24 toward each corresponding ring-shaped member 16 and intersect the yoke at right angles. The magnetic pole groups 26 and 30 face the upper portion of the upper ring-shaped member 16 and the lower portion of the lower ring-shaped member 16, respectively. A ring-shaped magnetic pole 34 is formed on that portion of the yoke 24 between the two magnetic pole groups 26 and 30. The ring-shaped magnetic pole 34 faces that portion of the cylindrical member 14 which extends between the two ring-shaped members 16. Ring-shaped permanent magnets 39 and 40 are arranged on those portions of the yoke 24 between the magnetic pole group 26 and the magnetic pole 34, and between the magnetic pole group 30 and the magnetic pole 34. Coils 28a, 28b, 28c, 28d, 32a, 32b, 32c and 32d (28b, 32b and 32d are not shown in FIG. 1) for adjusting the radial position of the supported body 12 are wound around the magnetic poles 26a, 26b, 26c, 26d, 30a, 30b, 30c and 30d, respectively. A pair of coils 36 and 38 wound around the yoke 24 are arranged individually on two flat surfaces of the ring-shaped magnetic pole 34. The coils 36 and 38 serve to adjust the longitudinal position of the supported body 12. The coils are energized, and magnetic fluxes delivered from the north pole of the permanent magnet 39 pass through the yoke 24, enter the ring-shaped members 16 via the magnetic pole group 26 and the magnetic pole 34, and return to the south pole of the permanent magnet 39, circulating in loops as indicated by broken lines in FIG. 1. Likewise, magnetic fluxes delivered from the permanent magnet 40 return thereto in loops indicated by the broken lines. For example, magnetic flux from the magnetic pole 26a enters the upper ring-shaped member 16 substantially at right angles to the peripheral surface thereof. Therefore, if the current to flow through the coil 28a is increased to raise the magnetic flux density, the supported body 12 moves toward the magnetic pole 26a. Thus, the radial position of the supported body 12 can be adjusted by regulating the currents to flow through the coil groups 28 and 32. Meanwhile, magnetic flux from the magnetic pole 34 enters the ring-shaped members 16 substantially at right angles to the lower and upper end faces of the upper and lower ring-shaped members 16 respectively. Accordingly, when the supported body 12 is located above a predetermined position thereof, the currents flowing through the coils 36 and 38 are increased and decreased, respectively, so that the magnetic fluxes from the magnetic pole 34 about to enter the upper and lower ring-shaped members 16 are intensified and weakened, respectively. Thus, the force of attraction between the upper ring-shaped member 16 and the magnetic pole 34 is increased to lower the supported body 12. The longitudinal and radial positions of the supported body 12 can independently be adjusted by regulating the coil energizing currents.
In the prior art electromagnetic bearing constructed in this manner, however, the magnetic fluxes flowing between the ring-shaped members 16 and the individual magnetic poles of the magnetic pole groups 26, 30 generally tend to concentrate at the edge portions of the magnetic poles. Thus, the magnetic fluxes are liable to be saturated at the edge portions. Saturation of the magnetic fluxes makes it impossible to control the radial position of the supported body 12. In consequence, it is hard to control the position of the supported body in a constant manner.
The density of the magnetic fluxes inside the ring-shaped members 16 is considerably higher in positions P1, P2, P3 and P4 near the individual magnetic poles of the magnetic pole group 26 than in positions Q1, Q2, Q3 and Q4 between the magnetic poles. If the magnetic flux distribution is such that the regions with high and low magnetic flux densities are alternately arranged along the circumferential direction of the ring-shaped members, the values of eddy currents produced on the rotating ring-shaped members are increased. These eddy currents dissipate part of the energy of the magnetic fluxes produced by the energizing currents, causing energy loss. The eddy currents increase in proportion to the change of magnetic flux density with respect to the time. Thus, if the rotational frequency of the rotating body becomes higher, then the eddy currents are increased in proportion. Therefore, the eddy currents attenuate the advantage of the electromagnetic bearing in permitting high-speed rotation. These drawbacks can be eliminated only by using a larger-sized motor with more power, that is, by increasing the size of the electromagnetic bearing.